Re: [PATCH 04/19] target/ppc: PMU Book3s basic insns count for pseries TCG

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Aug 17, 2021 at 06:30:37AM -0300, Daniel Henrique Barboza wrote:
> 
> 
> On 8/16/21 11:59 PM, David Gibson wrote:
> > On Mon, Aug 16, 2021 at 02:53:13PM -0300, Daniel Henrique Barboza wrote:
> > > 
> > > 
> > > On 8/10/21 12:39 AM, David Gibson wrote:
> > > > On Mon, Aug 09, 2021 at 10:10:42AM -0300, Daniel Henrique Barboza wrote:
> > > > > The PMCC (PMC Control) bit in the MMCR0 register controls whether the
> > > > > counters PMC5 and PMC6 are being part of the performance monitor
> > > > > facility in a specific time. If PMCC allows it, PMC5 and PMC6 will
> > > > > always be used to measure instructions completed and cycles,
> > > > > respectively.
> > > > > 
> > > > > This patch adds the barebones of the Book3s PMU logic by enabling
> > > > > instruction counting, using the icount framework, using the performance
> > > > > monitor counters 5 and 6. The overall logic goes as follows:
> > > > > 
> > > > > - a helper is added to control the PMU state on each MMCR0 write. This
> > > > > allows for the PMU to start/stop as quick as possible;
> > > > 
> > > > Um.. why does a helper accomplish that goal?
> > > > 
> > > > > 
> > > > > - only PMC5 and PMC6 are being set. PMC6 (cycles) is default to 4*insns
> > > > > (for cycles per instruction) for now;
> > > > 
> > > > What's the justification for that value?  With a superscalar core, I'd
> > > > expect average (median) cycles per instruction to be < 1 a lot of the
> > > > time.  Mean cycles per instruction could be higher since certain
> > > > instructions could take a lot.
> > > > 
> > > > > - the intended usage is to freeze the counters by setting MMCR0_FC, do
> > > > > any additional setting via MMCR1 (not implemented yet) and setting
> > > > > initial counter values,  and enable the PMU by zeroing MMCR0_FC. Software
> > > > > must freeze counters to read the results - on the fly reading of the PMCs
> > > > > will return the starting value of each one.
> > > > 
> > > > Is that the way hardware behaves?  Or is it just a limitation of this
> > > > software implementation?  Either is fine, we should just be clear on
> > > > what it is.
> > > > 
> > > > > 
> > > > > Since there will be more PMU exclusive code to be added next, let's also
> > > > > put the PMU logic in its own helper to keep all in the same place. The
> > > > > code is also repetitive and not really extensible to add more PMCs, but
> > > > > we'll handle this in the next patches.
> > > > > 
> > > > > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > > > > ---
> > > > >    target/ppc/cpu.h               |  4 ++
> > > > >    target/ppc/cpu_init.c          |  4 +-
> > > > >    target/ppc/helper.h            |  1 +
> > > > >    target/ppc/meson.build         |  1 +
> > > > >    target/ppc/pmu_book3s_helper.c | 78 ++++++++++++++++++++++++++++++++++
> > > > >    target/ppc/translate.c         | 14 ++++--
> > > > >    6 files changed, 97 insertions(+), 5 deletions(-)
> > > > >    create mode 100644 target/ppc/pmu_book3s_helper.c
> > > > > 
> > > > > diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
> > > > > index 4d96015f81..229abfe7ee 100644
> > > > > --- a/target/ppc/cpu.h
> > > > > +++ b/target/ppc/cpu.h
> > > > > @@ -1175,6 +1175,10 @@ struct CPUPPCState {
> > > > >        uint32_t tm_vscr;
> > > > >        uint64_t tm_dscr;
> > > > >        uint64_t tm_tar;
> > > > > +
> > > > > +    /* PMU registers icount state */
> > > > > +    uint64_t pmc5_base_icount;
> > > > > +    uint64_t pmc6_base_icount;
> > > > >    };
> > > > >    #define SET_FIT_PERIOD(a_, b_, c_, d_)          \
> > > > > diff --git a/target/ppc/cpu_init.c b/target/ppc/cpu_init.c
> > > > > index 71062809c8..fce89ee994 100644
> > > > > --- a/target/ppc/cpu_init.c
> > > > > +++ b/target/ppc/cpu_init.c
> > > > > @@ -6822,7 +6822,7 @@ static void register_book3s_pmu_sup_sprs(CPUPPCState *env)
> > > > >        spr_register_kvm(env, SPR_POWER_MMCR0, "MMCR0",
> > > > >                         SPR_NOACCESS, SPR_NOACCESS,
> > > > >                         &spr_read_pmu_generic, &spr_write_pmu_generic,
> > > > > -                     KVM_REG_PPC_MMCR0, 0x00000000);
> > > > > +                     KVM_REG_PPC_MMCR0, 0x80000000);
> > > > >        spr_register_kvm(env, SPR_POWER_MMCR1, "MMCR1",
> > > > >                         SPR_NOACCESS, SPR_NOACCESS,
> > > > >                         &spr_read_pmu_generic, &spr_write_pmu_generic,
> > > > > @@ -6870,7 +6870,7 @@ static void register_book3s_pmu_user_sprs(CPUPPCState *env)
> > > > >        spr_register(env, SPR_POWER_UMMCR0, "UMMCR0",
> > > > >                     &spr_read_pmu_ureg, &spr_write_pmu_ureg,
> > > > >                     &spr_read_ureg, &spr_write_ureg,
> > > > > -                 0x00000000);
> > > > > +                 0x80000000);
> > > > >        spr_register(env, SPR_POWER_UMMCR1, "UMMCR1",
> > > > >                     &spr_read_pmu_ureg, &spr_write_pmu_ureg,
> > > > >                     &spr_read_ureg, &spr_write_ureg,
> > > > > diff --git a/target/ppc/helper.h b/target/ppc/helper.h
> > > > > index 4076aa281e..5122632784 100644
> > > > > --- a/target/ppc/helper.h
> > > > > +++ b/target/ppc/helper.h
> > > > > @@ -20,6 +20,7 @@ DEF_HELPER_1(rfscv, void, env)
> > > > >    DEF_HELPER_1(hrfid, void, env)
> > > > >    DEF_HELPER_2(store_lpcr, void, env, tl)
> > > > >    DEF_HELPER_2(store_pcr, void, env, tl)
> > > > > +DEF_HELPER_2(store_mmcr0, void, env, tl)
> > > > >    #endif
> > > > >    DEF_HELPER_1(check_tlb_flush_local, void, env)
> > > > >    DEF_HELPER_1(check_tlb_flush_global, void, env)
> > > > > diff --git a/target/ppc/meson.build b/target/ppc/meson.build
> > > > > index b85f295703..bf252ca3ac 100644
> > > > > --- a/target/ppc/meson.build
> > > > > +++ b/target/ppc/meson.build
> > > > > @@ -14,6 +14,7 @@ ppc_ss.add(when: 'CONFIG_TCG', if_true: files(
> > > > >      'int_helper.c',
> > > > >      'mem_helper.c',
> > > > >      'misc_helper.c',
> > > > > +  'pmu_book3s_helper.c',
> > > > >      'timebase_helper.c',
> > > > >      'translate.c',
> > > > >    ))
> > > > > diff --git a/target/ppc/pmu_book3s_helper.c b/target/ppc/pmu_book3s_helper.c
> > > > > new file mode 100644
> > > > > index 0000000000..fe16fcfce0
> > > > > --- /dev/null
> > > > > +++ b/target/ppc/pmu_book3s_helper.c
> > > > 
> > > > I'd prefer to call this just book3s_pmu.c.  Or better yet
> > > > "powerX_pmu.c", where X is the specific PMU model you're implementing
> > > > since IIRC the particulars of the PMU vary quite a lot from POWER7
> > > > through to POWER10.
> > > 
> > > I'll go with book3s_pmu.c because this PMU implementation will not go
> > > deep enough to touch the differences between POWER processors.
> > 
> > I think you are mistaken.
> > 
> > > The only aspects that will be implementation specific will be 2 perf
> > > events (0x2 and 0x1E) that the kernel has been using for a long time
> > > and only recently migrated to architected events. We'll support all
> > > architected events that are related to those events so that newer
> > > kernels - and other non-IBM processors - will use the PMU without
> > > the need of having to know about 0x2 and 0x1E.
> > 
> > So, possibly in the last few POWER chips, the only differences are the
> > table of event keys.  That is definitely not the case for the whole
> > POWER line though.  I remember from my time at IBM, the PMU underwent
> > huge changes much deeper than the event table.  Some had different
> > numbers of PMCs, different bit fields in the MMCRs, different methods
> > of event selection (in some cases through insanely compplex chains of
> > multiplexers).  And everything from POWER4 onwards could reasonably be
> > described as "book3s".  So we definitely need a different
> > name... working out what it should be is harder though.
> > 
> > If the modern core structure of the PMU got codified in a particular
> > BookS architecture version we could name it after that version, maybe?
> 
> 
> What about just 'pmu_helper.c'? That way we can have a file with most of
> the PMU logic contained in it, without implying nothing about the support
> with the filename alone. If time goes by and more specialized coded starts
> to being added in there (code like "this helper implements something that
> is only valid in PPC chip X") then we can split into more specific files.
> 
> Yet another alternative in line with what you suggested could be
> "power9_pmu.c", perhaps even "power8_pmu.c". The chip version would mean
> "the oldest IBM TCG PPC64 chip that we tested with this PMU code". I'm
> testing with POWER9 most of the time, POWER10 works fine, so 'power9_pmu.c'
> is viable. I can do some tests with POWER8 to see how it goes. I'm not
> sure if it's worth the trouble testing with anything older than P8
> though.

Right.  I much prefer either of these to pmu_helper.c

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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